Pancreatic Disease (PD) is a serious disease that affects fish, in particular salmonid fish such as wild Atlantic salmon, rainbow trout and the like. The disease causes lesions in the pancreas, including loss of pancreatic exocrine tissue, and fibrosis, cardiac and skeletal muscle myopathies. Outbreaks of PD were first described in 1984 by Munro et al, in Helgoland Meeresuntersuchungen 37:571-586 (1984). PD typically affects the fish post-molts during the first year after they are transferred to sea sites and is reported to spread rapidly among farm fish held in sea cages. Clinical signs include lethargy with a tendency to congregate in cage corners and to fail to maintain a horizontal position, cessation of feeding (anorexia) and significant mortalities (Ferguson et al, J. Fish Disease 9:95-98, 1986). Murphy et al (in J. Fish Disease 15:401-408, 1992) confirmed these observations in a later study, in which it was found that cardiac and skeletal myopathy is exacerbated in fish suffering from PD.
An outbreak of PD in a fish farm can cause growth to be reduced and up to 10 percent of surviving fish may prove to be runt. On Irish fish farms PD causes significant mortality rates of 10 to 60 percent among the young fish during the first year after they are transferred to sea sites (McLoughlin, M., Fish Farmer page 19, March/April 1995). The estimated cost to the Irish industry in terms of loss of production is currently thought to be around £25 million per year. Consequently, there is a great need for a vaccine for the prevention and/or treatment of PD in fish.
EP-A-712926 describes the isolation of the causative agent of PD from tissues of PD affected fish arid the identification of the virus as a toga-like virus. To prevent PD infections in fish, the use of attenuated or inactivated PD for vaccination of the fish is accordingly suggested. A drawback in the production of inactivated vaccines from the PD virus described in EP-A-712926 is the slow growth of the virus, in particular on cell cultures, which makes the manufacturing of said vaccines a relatively inefficient process. A further drawback with the inactivated vaccines is the instability of the inactivated virus in the presence of other inactivated pathogens resulting in potency loss. Fish vaccines are generally produced as multivalent vaccines, and significant higher amounts of inactivated virus are required in the multivalent vaccine than would be necessary in a monovalent vaccine to compensate for the loss of potency.